Random Thoughts: SSTOs
Ok, I asked people to put up suggestions on my Skribit account, and I figured it was about time to actually start responding to some of the suggestions. One of the topics someone asked about was if the Saturn S-IVB could be turned into an SSTO launch vehicle. While looking at the mass ratio, it may appear pretty close to an SSTO, it would've required a completely diferent propulsion system. The T/W ratio at takeoff would've been less than 1, and more importantly, the J-2 was designed for vacuum operations, not surface operations. So, no, the S-IVB wouldn't have made a good SSTO without a lot of work.
That isn't to say that there haven't been numerous nearly SSTO stages that have flown throughout the years. The problem is that an expendable SSTO just really isn't that useful. Sure, its less complex than a TSTO vehicle, but since you're throwing the bird away after every flight, and since the payload to dry mass ratio for such a system is likely going to be a lot worse, I'm skeptical that there's any practical advantage. If there is, it would be a modest one at best.
No, SSTO really only makes a big difference if it's combined with reusability. Now, while the conventional wisdom may say that an SSTO RLV is impossible, I wouldn't go that far. They're not impossible at all. In spite of claims to the contrary, there really have been technological improvements in rocketry over the past 50 years that make such ideas more practical now than then. Electronics are much lighter now. There are new composite materials that really can (when properly used) reduce weight substantially. There are propulsion advances like the different forms of altitude compensation, and Thrust Augmented Nozzles that can change the mission-averaged Isp and engine T/W ratio of rockets drastically. More to the point, there still are several dimension of performance for which the narrow evolutionary path of rockets have barely scratched the surface. There's no physics-based reason why even ground-based SSTO RLVs couldn't work based on currently discovered engineering concepts and principles.
My argument against most SSTO approaches is more a pragmatic one than a dogmatic one. Namely, while SSTO RLVs aren't impossible they are premature. Other than the airlaunched "assisted SSTO" concept, I don't think any of the other SSTO RLV approaches are close enough to our current state of the art to be worth trying at this point. There's enough work we still need to figure out to get any RLVs flying reliably, that we really don't need to be making our lives more difficult up front. And for all the complaints about the operational drawbacks of TSTO approaches compared to SSTO approaches, even a working TSTO RLV would be such an improvement over the state of the art that it would be revolutionary. Sure, there's going to be some point in the future where everyone is doing TSTO RLVs, and the hassles of having to remate stages before you can turn them around is going to be enough of a limiter on flight rate that taking that next step to SSTO will make plenty of sense. But those are problems that I really wish we had.
In other words, sure SSTO is possible, it's just probably not the right problem to be solving right now.
That isn't to say that there haven't been numerous nearly SSTO stages that have flown throughout the years. The problem is that an expendable SSTO just really isn't that useful. Sure, its less complex than a TSTO vehicle, but since you're throwing the bird away after every flight, and since the payload to dry mass ratio for such a system is likely going to be a lot worse, I'm skeptical that there's any practical advantage. If there is, it would be a modest one at best.
No, SSTO really only makes a big difference if it's combined with reusability. Now, while the conventional wisdom may say that an SSTO RLV is impossible, I wouldn't go that far. They're not impossible at all. In spite of claims to the contrary, there really have been technological improvements in rocketry over the past 50 years that make such ideas more practical now than then. Electronics are much lighter now. There are new composite materials that really can (when properly used) reduce weight substantially. There are propulsion advances like the different forms of altitude compensation, and Thrust Augmented Nozzles that can change the mission-averaged Isp and engine T/W ratio of rockets drastically. More to the point, there still are several dimension of performance for which the narrow evolutionary path of rockets have barely scratched the surface. There's no physics-based reason why even ground-based SSTO RLVs couldn't work based on currently discovered engineering concepts and principles.
My argument against most SSTO approaches is more a pragmatic one than a dogmatic one. Namely, while SSTO RLVs aren't impossible they are premature. Other than the airlaunched "assisted SSTO" concept, I don't think any of the other SSTO RLV approaches are close enough to our current state of the art to be worth trying at this point. There's enough work we still need to figure out to get any RLVs flying reliably, that we really don't need to be making our lives more difficult up front. And for all the complaints about the operational drawbacks of TSTO approaches compared to SSTO approaches, even a working TSTO RLV would be such an improvement over the state of the art that it would be revolutionary. Sure, there's going to be some point in the future where everyone is doing TSTO RLVs, and the hassles of having to remate stages before you can turn them around is going to be enough of a limiter on flight rate that taking that next step to SSTO will make plenty of sense. But those are problems that I really wish we had.
In other words, sure SSTO is possible, it's just probably not the right problem to be solving right now.
Labels: Launch Vehicles, Space Transportation, Technology
posted by Jon Goff at 9:23 PM
9 Comments:
SSTO RLV is a red herring for now. I don't know if the concept has done more harm than good to RLV:s.
Gravityloss,
Yeah, it probably is a red herring at the moment. SSTO may have some benefit over TSTO, but TSTO RLVs have so much to offer compared to ELVs. As I said--premature.
~Jon
If someone re-built a Centaur upper-stage with modern composite technology and the same RL10 engines, one could probably have an SSTO that could reach orbit after being dropped from Rutan's White Knight 2.
It costs less than $5 Million for the RL10 engines, and XCOR or Scaled Composites could probably build the composite upper-stage/SSTO for under $50 Million including the test launch from WK2.
Rutan and other business people probably already know this, but they are waiting to see what happens with SpaceShip Two, because they are looking at this as a business instead of as a sceince project.
Anonymous
Phil Bono published a Wonder Book-sized book 30 years ago about several of his designs. It included SSTO RLV designs. An entire chapter, as I recall, was devoted to a project called SASSTO (Saturn Applications Single Stage to Orbit). It was to use the annular aerospike derivative of the J-2 (see http://www.astronautix.com/lvs/sassto.htm ) Engine could have been J-2T (see http://www.astronautix.com/engines/j2t250k.htm), can't be sure since I lost my copy from when I was a kid during a move years ago (argh)
If I remember correctly, the SII stage could theoretically reach orbit as an SSTO with a payload of 10,000+ pounds; again, though, it would need significant changes to work in the real world (e.g. probably an SASSTO-style aerospike engine). Atlas, obviously, was almost an SSTO, since it was a single stage that dropped two engines on the way to orbit.
While it's certainly true that the big benefits of SSTO would only come from a fully reusable vehicle with a high flight rate, there could still be significant benefits to a throwaway SSTO; assembling a rocket from multiple stages and then separating them at hypersonic speeds and starting the engines in flight is still not simple and cheap.
You can see the discussion about the S-IVB in an appendix to this paper:
http://www.spacefuture.com/archive/history_of_the_phoenix_vtol_ssto_and_recent_developments_in_single_stage_launch_systems.shtml
The appendix itself is here:
http://www.spacefuture.com/archive/a_single_stage_to_orbit_thought_experiment.shtml
I later calculated, but did not publish, what would happen if one substituted high pressure hydrocarbon engines (i.e., a rubber RD-170 or equivalent) for the SSME. The payload doubled from 10K to 20K lbs.
Redefine "SSTO."
The simplest way is to use a drop tank (external tank) to carry the boost phase propellant, while the orbiter is otherwise the entire rest of the vehicle, including all the propulsion system.
An external tank dropped at, say, Mach 5-6 and 200-250K feet would be "staging" at roughly the same speed and altitude as the Shuttle SRBs. Ocean recovery of the drop tank would be simpler and less expensive than with the SRBs, as it would just be an empty propellant tank. No re-entry shielding or fly-back requirements.
This "redefinition" variant would be the simplest way to deal with the weight-penalty issues that have bedeviled programs like X-33. Just drop some recoverable and refurbishable weight. It's a fully reusable system with a simple, tried and tested recovery system for the external tank, and the fly-back technique used by the Shuttle. Also, by dropping an external tank, it minimizes the size of the orbiter, making re-entry and fly-back less problematic.
Also, such a concept might be enhanced further using a ring insert in the engine nozzles for a low-altitude-efficient expansion ratio through the boost phase. The ring insert (basically a device of similar construction and material as a solid rocket motor nozzle) would be expelled at the end of the boost phase, say, shortly after losing the external tank. The orbiter would proceed with it's wider nozzle expansion for altitude and using its on-board propellant.
If you can build a reusable First Stage Booster, you might be in spitting distance of a SSTO. A VTVL Reusable first stage that gets your upper stage 3-400 KM up and going say mach 3 Downrange makes the rest of the problem probable. THe trick is you have to size the booster to either Fly Straight vertical and fall back to the base, or you need to size the booster to fall downrange, and be transportable by a barge, vehicle or airplane.
Anonymous,
Before we go on, I wanted to mention that I've moved the blog over to www.selenianboondocks.com. This blogspot blog will eventually go away. Second, I wrote a series on Orbital Access Methdologies that goes into alternatives for first-stage recovery/RTLS. There are actually a lot of options out there, though I only detailed a few of them.
~Jon
Post a Comment
<< Home